A representative state of the art is known from German OS No. 33 04 98 (which corresponds to U.S. application Ser. No. 816,946, filed Jan. 6, 1986) which is not prepublished. Cubic boron nitride (CBN) is used for tools for the precision working of the flanks of gears, as they are described in this reference, just like in many other cases, and in which boron nitride is applied, for example galvanically, to the working surfaces, thus the tooth flanks, of a metallic base member. The machining operation results which are achieved with such tools are generally very satisfying. However, with respect to the edge life, these tools do not always meet the expectations of the user, since often a premature breaking out and a strong wear of the CBN-granules is noticed. The reason for this was first unknown, until it was discovered that the CBN requires a specific minimum cutting speed in order to bring about satisfactory work results over long periods of time. Thus, it is not possible to speed up the tool to the necessary cutting speed while it is already in engagement, because it has then already started machining at a cutting speed which is too low and the CBN-coating is thereby destroyed. This is valid both for the active, that is chip-removing use of the tool, and also the passive use, that is during a dressing of the tool.
Therefore, the basic purpose of the invention is to further develop the abovementioned method in such a manner that the mentioned difficulties no longer occur. The method is also to be usable for a dressing of the tools.
With respect to the precision working of toothed workpieces, the purpose is attained by:
(a) moving the tool into the tooth system of the workpiece; or vice versa;up and until a center distance corresponding with the start of the machining operation is attained;
(b) rotating the tooth system of one of the two parts of the workpiece-tool pair relative to the tooth system of the guide-gear pair until the machining tool flank on the one side engages the workpiece flank which is to be machined and on the other side engages the corresponding mating flank of the guide-gear pair;
(c) moving the tool with its guide-gear on the one side and the workpiece with its guide-gear on the other side away from one another while maintaining the flank contact in the guidegear pair up and until a condition whereat backlash exists on both sides of the tool tooth;
(d) activating the rotary movement and accelerating to the working speed;
(e) vertical feeding the tool relative to the workpiece (reducing the center distance) at a simultaneous chip removal, whereby the tool is guided by the guide-gear pair so that only one flank of the tool teeth engages the workpiece teeth;
(f) moving the tool and the workpiece apart, while the guiding is maintained by the guide-gear pair, and then deactivating the rotary movement;
(g) repeating the operation for the other tooth side of the workpiece and the tool.
It is possible with a method which is developed in this manner to bring the tool which is coated with CBN first to the necessary cutting speed before the machining operation starts. For reasons of economy, the tool should be dressed in the same machine in which it is being used. The tool which is coated with CBN must then be dressed with a dressing gear which is coated with diamonds. Thus far, the additional problem was created that the granules of the tool and the dressing gear claw into one another so that the granules are torn out. If, however, the method according to the invention is carried out utilizing granules on the dressing wheel that are harder than the CBN-coating, then the CBN-coating is substantially protected.
The dressing of the tool can occur in a very advantageous manner by:
(a) moving the tool which is to be dressed into the tooth system of the dressing wheel or vice versa;
(b) reducing the center distance to the smallest center distance of tool and dressing wheel;
(c) rotating the tooth system of one of the two parts of the tool-dressing-wheel pair relative to the tooth system of the guide-gear pair until the machining dressing-wheel flank engages the tool flank to be dressed on the one side and the corresponding mating flank of the guide-gear pair engage on the other side and backlash exists on the respective other flanks;
(d) moving the tool with its guide-gear on the one side and the dressing wheel with its guide-gear on the other side apart while maintaining flank contact in the guide-gear pair up and until a condition whereat backlash exists on both sides of the tool tooth;
(e) activating the rotary movement and accelerating to the working speed;
(f) vertically feeding the tool relative to the dressing wheel (reducing the center distance) with a simultaneous dressing of the tool flanks whereby the tool is guided by the guide-gear pair in such a manner that only one flank of the tool teeth engages the dressing-wheel teeth;
(g) moving the tool and the dressing wheel apart while guiding is maintained by the guide-gear pair and then deactivating the rotary movement;
(h) repeating the operation for the other tooth side of the tool and the dressing wheel.